Carbon nanotubes can be classified by the number of walls in the tube, single-wall, double wall and multiwall. Each wall of a carbon nanotube can be further classified into chiral or non-chiral forms. Carbon nanotubes are currently manufactured as agglomerated nanotube balls or bundles. Use of carbon nanotubes and graphene as enhanced performance additives in batteries is predicted to have significant utility for electric vehicles, and electrical storage in general. However, utilization of carbon nanotubes in these applications is hampered due to the general inability to reliably produce individualized carbon nanotubes.
The performance goals of a lead-acid battery are to maximize the specific power (power per unit of weight, measured in watts per kilogram) over designated high rate discharge scenarios, and maximize battery life, not only in environmental durability but also most importantly in cycle life (number of possible charges, and discharges).
Both corrosion (on the positive plate) and sulfation (on the negative plate) define two key failure modes of today's lead acid batteries. Regarding corrosion failures, this failure mode begins to accelerate either as temperatures rise about 70° F., and/or if the battery is left discharged. To mitigate the effects of the corrosion process, most battery companies focus their research on developing more corrosion resistant lead-alloys and grid manufacturing processes that reduce the mechanical stresses in the as-manufactured grids. Regardless of the alloy or grid fabrication process, essentially all battery manufacturers engineer battery service life based on lead alloy and grid wire cross-sectional area. Normally this engineering translates as a change in grid thickness and corresponding plate thickness. Thicker grids provide longer life, but usually sacrifice power density, cost, weight, and volume.
Regarding sulfation failures, when a lead acid battery is left on open circuit stand, or kept in a partially, or fully discharged state for a period of time, the lead sulfate formed in the discharge reaction recrystallizes to form larger, low surface area lead sulfate crystals which are often referred to as hard lead sulfate. This low surface area, non-conductive lead sulfate, blocks the conductive path needed for recharging. These crystals, especially those furthest removed from the electrode grid, are difficult to convert back into the charged lead and lead dioxide active materials. Even a well maintained battery will lose some capacity over time due to the continued growth of large lead sulfate crystals that are not entirely recharged during each recharge. These sulfate crystals, of density 6.287 g/cc, are also larger in volume by about 37% than the original paste, so they mechanically deform the plate and push material apart. The resulting expansion and deformation of the plates also causes active material to separate from the electrodes with a commensurate loss of performance. Sulfation is the main problem in recreational applications during battery storage when the season ends. Boats, motorcycles, snowmobiles lie dormant in their off-use months and, left uncharged, discharge toward a zero % state-of-charge, leading to progressive sulfation of the battery. Thus, the battery cannot be recharged anymore, is irreversibly damaged, and must be replaced.
As users have come to know portable battery products in cell phones and laptop computers, they have correspondingly become comfortable with the process of bringing a battery down to almost no charge and then bringing it back to full, complete charge and power capabilities within hours. Traditional lead-acid batteries, because of their inherent design and active material utilization limitations, only provide relatively good cycle-life when less than about 80% of the rated capacity is removed during each discharge event in an application. A battery of this type suffers a significant decrease in the number of times it can be discharged and recharged, i.e., cycle life, when 100% of the rated capacity is consumed during a single discharge in an application. Many new products that historically used lead-acid batteries are requiring a significant jump in cycle life. The most notable examples are Hybrid Electric Vehicles, which operate in a High Rate Partial-State-of-Charge condition. This is a punishing application which dramatically shortens the cycle life of a typical lead acid battery, and has therefore left car companies with no choice, but to go to much more expensive Nickel-Metal Hydride batteries, and experiment with Lithium ion batteries.
Typically, a lead-acid battery will require a recharge time significantly longer than competitive batteries containing advanced materials seen in portable products. A complete charging of a lead-acid battery, such as found in electric vehicles, can take from 8 to 16 hours. In the case of Uninterrupted Power Supplies (UPS), a rapid charge rate is essential to ensuring quality performance, as well as reducing the related capital expenditures for back up equipment while charging takes place on initial batteries put into service.
Environmental conditions such as vibration can also result in degradation of a lead-acid battery due to active material separating from the cathode or anode. More vibration-resistant batteries, such as used for pleasure boats, often contain thicker electrodes or special vibration damping structures within the battery. This increases the weight and cost of the battery. Hence, an increased mechanical strength of the active material paste would be a highly desirable feature.
Traditional methods for producing battery plates for lead-acid batteries generally involve a mixing, curing and drying operation in which the active materials in the battery paste undergo chemical and physical changes that are used to establish the chemical and physical structure and subsequent mechanical strength necessary to form the battery plate. To produce typical battery plates, materials are added to commercial paste mixing machines common in the industry in the order of lead oxide, flock, water and sulfuric acid, which are then mixed to a paste consistency. The flock component is a fibrous material, usually composed of polyester, nylon or acrylic fibers, which is added optionally to the paste to increase the mechanical strength of the pasted plate. An “expander” component is conventionally added to the negative paste consisting of a mixture of barium sulfate, carbon black and lignosulfonate that is added to the negative paste to improve the performance and cycle lifetime of the battery. During mixing, chemical reactions take place in the paste producing basic lead sulfates, the most common of which is tribasic lead sulfate. The final paste composition is a mixture of basic lead sulfates, unreacted lead monoxide and residual free lead particles. Pasting is the process of making a battery plate. This paste is dispersed into a commercial automatic pasting machine of a type common in the industry, which applies the paste to a grid structure composed of a lead alloy at high speed. The paste plates are generally surface dried in a tunnel dryer of a type common in the industry and then either stacked in columns or placed on racks. The stacked or racked plates are then placed in curing chambers. It is very important during the entire pasting and curing operation that the paste has sufficient mechanical strength to avoid micro-crack formation and hence increased internal electrical resistance from the paste mix. A high internal electrical resistance can limit rates of discharge and charging as well as result in localized heating during charging/discharging and increased chemical degradation of the active species.
In efforts to reduce the high impedance of the battery to accelerate the formation (first charging) step, carbon black has been added to the paste. However, to properly disperse the carbon black surfactants are employed, but these surfactants create higher impedance that is difficult for the carbon black particles to reduce. Also, because there is often a region of high impedance due to the non-homogeneous contact resistance of the powders there is often applied an overvoltage which results in electrolysis of water, generating oxygen at the cathode which then rapidly degrades the carbon black. It is highly desirable to have a means to lower impedance in lead-acid batteries that can avoid overvoltage requirements for charging as well as a longer lasting conducting additive for the cathode.